1. Field of the Invention
This invention relates generally to the forming composite components and relates specifically to forming composite components in a resin-transfer molding process.
2. Description of the Prior Art
Composite parts can be fabricated in a closed-mold process called resin-transfer molding (RTM), which is a commonly-used fabrication technique. Some RTM-produced parts are very simple in geometry, however recent developments have demonstrated RTM production of complex, unitized structures. Currently, commercially-available product forms such as woven and braided carbon fiber products are used in RTM production. As shown in FIG. 1, fabric 11 is typically woven from fibers 13, 15 oriented at a selected angle relative to each other, for example +/−45° or +/−90°. These products limit the design of many complex, load bearing, composite structures. The strength and stiffness of braided- and woven-fabric-based composites are inferior to unidirectional fiber designs. This is attributed to the fiber “waviness,” caused by fibers weaving over and under fibers in the same plane and shown in FIG. 1, and also attributed to lower fiber volume fractions attainable with braided or woven products.
A typical RTM tool requires stacking many layers of carbon fabric inside a mold. Carbon fabric is used because it can be easily handled, cut to shape, and laid in place without disturbing the designed orientation of the individual carbon fibers. It is critical to orient the individual carbon fibers, hence the fabric, in the manner specified by the design engineer to ensure proper load capacities will be satisfied. Sometimes bundles of stitched fabric layers or braided preforms are used in RTM production, though these are expensive and difficult to manufacture.
As shown in FIGS. 1 through 3, RTM is typically done by placing fabric 11 into a tightly-sealed, matched, metal mold 13 and then injecting resin into mold 13. FIG. 2 shows a cross-section of a RTM mold 13, which comprises an upper portion 19 and a lower portion 21. Lower portion 21 has injection ports 23 and vent ports 25 extending through the thickness of lower portion 21. Ports 23, 25 lead to a cavity 27 located between mold surface 29 of upper portion 19 and mole surface 31 of lower portion 21.
As shown in FIG. 3, fabric 11 is laid on mold surface 31 of lower portion 21, and upper portion 19 is secured in position on lower portion 21, forming cavity 27 around fabric 11. Resin (not shown) is injected into cavity 27 through each injection port 23 while a vacuum is pulled on each vent port 25. The vacuum helps to pull the resin throughout fabric 11 and minimize dry spots formed by air pockets. After resin injection is complete, ports 23, 25 are closed, and mold 13 is heated to cure the resin. After cure, mold 13 is cooled and disassembled to release the composite part.
Two additional RTM methods for forming composites are reaction injection molding (RIM) and vacuum-assisted resin-transfer molding (VARTM). The RIM process uses the same type of mold as RTM, such as mold 13 in FIGS. 2 and 3, but a two-part resin is mixed immediately prior to injection, which may eliminate additional steps required for curing of the composite components.
The VARTM process, illustrated in FIG. 4, uses a one-sided negative or positive mold, here illustrated as the lower portion 21 of mold 13 (FIGS. 2 and 3), and a vacuum bag 32 enclosing mold cavity 33. Layers of fabric 11 are laid on mold 21 as described above, then vacuum bag 32 is placed over fabric 11 and mold 21. Air within cavity 33 is evacuated, and vacuum bag 32 compresses fabric 11 against surface 31 of mold 21. Springs, mesh, or other stiffeners may be used in certain regions to maintain resin flow paths. As in the RTM process, resin is then injected into cavity 33 through each injection port 23 while a vacuum is pulled on each vent port 25. Though ports 23 and 25 are shown in lower portion 21, injection and vent ports may also be located in vacuum bag 32.
It is useful to make comparisons to other, traditional manufacturing methods and materials. The simplest type of composite production techniques is hand layup. Parts that are laid by hand can be made of unidirectional tape, in which all fibers run parallel to each other. Use of unidirectional fabric in hand layup presents a benefit over typical RTM production because of the higher load-carrying capacity for a given composite laminate weight. The specific modulus and the specific strength of composites made from unidirectional tape are greater than for composites made from woven fabric prepreg. This translates into thinner, lower-weight components when unidirectional prepreg tape is used.
However, unidirectional prepreg tape cannot be used in an RTM production process without complications. Since prepreg tape already contains resin which is semi-cured, this resin will complicate the resin flow in the RTM mold. Also, complex unitized structures create a maze-like resin flow path. The probability of success in complete part wet-out for production of a high-quality part is increased by maximizing possible resin flow paths.
Dimensional stability is also difficult to control when a net shape, closed mold process is used. Prepreg tape is stiff at room temperature and presents problems when closing a matched metal mold. Higher fiber volume fractions are possible if the matched metal mold is closed with only carbon fiber inside, which can translate into higher component performance. However, fibers can move relative to each other and to the mold, leading to degradation of performance.
Some unidirectional “fabric” products exist. These materials are predominantly unidirectional carbon fiber stitched together at regular intervals with a fill direction yarn. FIG. 5 shows a portion of unidirectional fabric 34, with fibers 35 being held together by threads 37. Threads 37 are ignored for load-bearing considerations and are only intended to hold together the unidirectional, wrap direction fibers. These products are difficult to handle and present complications with maintaining alignment of the fibers, especially over complex contoured surfaces. Once a ply of fabric 34 is laid down, it can be disturbed by layup of a subsequent ply. Unidirectional fabrics are difficult to cut to net shape and the cut edges often become ragged.
Thus, there is a need for a method of producing composite components in an RTM process using unidirectional fibers that avoids the resin flow path problems caused by using unidirectional prepreg tape and provides for maintaining alignment of fibers during layup.